This invention relates generally to polarized fusion reactors, i.e. reactors employing polarized fuel; and, more particularly, to reactor first-walls and first-wall coatings.
Kulsrud, Furth, Valeo, and Goldhaber have shown and described in U.S. patent application Ser. No. 492,924, filed May 9, 1983 how the performance of magnetic confinement fusion reactors can be improved by using deuterium and tritium plasmas that are polarized in certain preferred directions. They showed that polarizing the plasma fuel ions enhances the different nuclear fusion rates and controls the angular distribution of emitted reaction products. The feasiblity of using polarized plasmas depends on the absence of physical mechanisms that can depolarize the plasma rapidly on the timescale of particle confinement (.about.1 sec) or fuel burnup (.about.100 sec). Kulsrud et al demonstrated that the depolarization mechanisms in the plasma are too weak to be relevant.
There are, however, other depolarization mechanisms present which have not been considered carefully, and which are more important than those occurring in the plasma. These arise from the recycling of D and T at the limiter and the first wall of the reactor vessel. In all magnetic confinement scheme studies to date, the D-T fuel leaks out across the magnetic field lines about 10 to 20 times faster than the rate of fuel consumption. (Indeed, for a burning plasma, the ions must leak out at an appreciable rate in order for the helium ash to leave the plasma core.) As a consequence, most particles leave the plasma, strike the limiter (or first wall) and reenter the plasma about 20 times before fusing or before being pumped away. About half the flux of particles striking the first wall is reflected back into the plasma. The remaining particles have sufficient kinetic energy (20-100 eV) to penetrate about 100 .ANG. into the wall. There the D and T ions come to equilibrium and eventually diffuse back to the surface where they recombine and desorb as molecules, or are desorbed directly by ion, electron, and photon bombardment.
The D and T nuclei in the crystal lattice and in the vicinity of the wall surface are subject to depolarization mechanisms that are not present in the plasma core. In the plasma edge and in cracks or voids of the wall material, D and T can exist as polyatomic molecules. For these species, tumbling caused by rotation and collision of the molecules can flip the nuclear spins. For absorbed D and T residing in the crystalline lattice, depolarization can occur by thermal diffusion and by photon and electron induced fluctuations of microscopic magnetic fields and electric field gradients (which couple to nuclear magnetic dipole and electrical quadrupole moments respectively).
These new mechanisms can greatly reduce the nuclear magnetization of D and T on timescales of 10.sup.-3 and 10.sup.1 seconds. Since the mean residence times of absorbed D and T in the wall are also comparable with these timescales, a careful choice of the composition and temperature of the first wall and limiter is crucial in preventing rapid depolarization of the plasma.
Therefore, it is an object of the present invention to provide a first-wall or first-wall coating which prevents rapid depolarization of a polarized plasma.
It is another object of the present invention to provide a method of preventing (or minimizing) rapid depolarization of a polarized plasma in a fusion reactor.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.